There's also the expensive repaints of some of the ones you already mentioned... Metrotitan and Grand Maximus (plus Brave Maximus, the stripped-down RiD Fort Max repaint).

As for favorites, I'm a big Metroplex fan. He's a fun, affordable figure with lots of bells and whistles. I also dig Scorponok, though I don't own the mold and haven't seen it in person since I was a kid.

Micromaster bases are pretty easy to find and cheap. You can find the smaller ones such as Hot House for around $10 USD and larger ones like Countdown for around $20-$50 USD loose. It's also great that you can use the ramps to connect the bases together. I have all of them except Groundshaker, and they are all very cool

only HTF micromaster US bases to find are unbroken skystalkers and the missile launcher/tanker transports the latter can set you back 70-100$ depending on condition (at least on ebay)
how I wish Takra re-releases grandus and big powered they had some neat base modes

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